1. Field of the Invention
The present invention generally relates to protection of electronic devices from electrical overstresses and, more particularly, the present invention relates to the protection of electronic devices from overstress transients with extremely fast rise times and high peak power.
2. State of the Art
It is well known that electronic circuitry must be protected from transient voltage and current conditions which exceed the capacity of the circuitry. Such electrical transients can damage circuitry and can cause errors in operation. Particularly, protection from electrical overstress disturbances is required for modern electronic communication and control systems whose solid-state microelectronic components are highly sensitive to excessive currents and voltages.
Various devices and methods are known for providing protection from limited electrical overstresses. At the most basic level, it is common to shield electronic devices from electromagnetic transients with grounded wire screen enclosures. Such shielding, however, does not protect electronic devices from transient electrical overstress disturbances which penetrate into shielded circuits via connecting conductor lines. To protect circuitry from such transient overstress disturbances, a variety of protective devices are conventionally used singularly or in combination. Such devices include fuses, spark gaps, varistors, zener diodes, transzorbs, thin-film devices, bypass capacitors, inductors and filters. These devices are often referred to as voltage suppressors or voltage arrestors, but can be generally described as electrical overstress (EOS) protection devices. In use, EOS protection devices are connected between a circuit to be protected and ground, or between a conducting line leading to a circuit to be protected and ground. Their purpose is to shunt EOS transients to ground before energy resulting from the transients can damage protected circuitry.
For present purposes, an EOS transient can be defined as a transient voltage or current condition that can damage or upset normal operation of circuits. Electrical overstress transients of practical concern may arise from an electromagnetic pulse (EMP), lightning, or an electrostatic discharge (ESD). Such transients may rise to their maximum amplitudes in periods ranging from less than a few nanoseconds to several microseconds, and may be repetitive. In the rollowing, EOS transients are sometimes also referred to as pulses and surges.
A common example of an ESD overstress transient arises when there is a build-up of static electricity on persons wearing insulating clothing in carpeted offices. The ESD transients at time of discharge can include voltages exceeding 20,000 volts and currents of more than 40 amperes; such transients can upset or destroy electronic components in computers and other electronic devices. ESD transients may reach peak discharge voltages in less than a few nanoseconds and, as such, are faster than conventional overstress protection devices.
Lightning is another example of an EOS transient capable of adversely affecting electronic circuits. A lightning strike as near as several miles can radiate sufficient electromagnetic energy to generate pulse amplitudes of several thousand volts on power lines. Typically, the time to peak of lightning-caused transients is several microseconds and, thus, such transients are several thousand times slower than ESD transients.
EMP transiants are generated by nuclear weapons or other high-energy directed devices. A nuclear explosion, for example, may generate electric fields in excess of 50,000 volts per meter over a radius of more than 600 miles. The peak amplitudes of such fields can be reached in a few nanoseconds and the resulting EOS transients can disable communication equipment as well as other electronic devices.
EMP-caused threats to microelectronic components, especially junction field effect transistors and microwave diodes, are discussed by H. R. Philipp and L. M. Levinson in an article entitled "NbO Devices for Subnanosecond Transient Protection", J. App. Phys. 50(7). July 1979. The authors emphasize that conventional devices are intended to protect power or low-frequency circuits against lightning or switcning surges, and do not provide adequate protection against fast rise-time EMP transients. (The rerm "rise time" refers to the time required for a transient to reach maximum amplitude.)
A simple example of a device to protect against electrical disturbances is an ordinary fuse. Fuses are sensitive to current flow in power lines and, in high current situations, are heated to the point of rupture; after rupture, fuses create open circuit conditions. Because heating requires significant time, fuses are not acceptable in situations where extremely rapid responses are required. For example, fuses do not adequately respond to EOS transients with rise times of a few microseconds. In addition, fuses are unacceptable in many electrical overstress protection situations because, after responding to an EOS condition, fuses irreversibly and destructively break down and must be replaced. A more desirable property would be for fuses to automatically recover their protective abilities after protecting against an EOS transient.
In fact, the ability to automatically recover protective properties is available to some extent in many conventional overstress protection devices, particularly varistors. Varistors usually have a characteristic known as a "clamping" voltage. For applied voltages below the clamping value, a varistor provides high resistance and, therefore, essentially acts as an open circuit. On the other hand, for applied voltages which substantially exceed the clamping value, a varistor provides substantially reduced resistance to shunt high-amplitude electrical transients to ground. Accordingly, when a varistor is connected to a line carrying signals, the varistor will not affect signals on the line at ordinary voltage levels but will shunt high amplitude EOS disturbances, at least ones with relatively slow rise times.
The property of exhibiting high resistance at voltages below a clamping level and low resistance at voltages above the clamping level will be referred to herein as non-linear resistance (NLR). Various materials are known to have NLR properties; a common example is zinc oxide. Such materials are used in numerous overstress protection devices; for example, varistors are often fabricated from zinc oxide particles. When such materials are in a high resistance state, the materials are said to be in the "off-state"; when the materials are in a low resistance state, the materials are said to be in the "on-state".
Varistors are commercially available with sufficient capacities to provide overstress protection against relatively large amounts of transient energy such as encountered in lightning surges. However, one shortcoming of varistors is their relatively high capacitance, which delays response times. The structure and operation of varistors is described in "The Transient Voltage Suppression Manual", fourth edition, published in 1983 by General Electric Company, U.S.A. According to the manual, varistors have the capacity to handle up to 200 joules of energy with current flows of up to 6000 amperes. The microstructure of varistor material is comprises of grains of sintered metal oxide powder having the property that the voltage drops across intergranular boundaries are nearly constant, usually at about 2-3 volts per grain boundary junction, independent of grain size.
A particular varistor material is suggested in U.S. Pat. No. 4,103,274. According to this patent, a varistor can be fabricated from polycrystalline meral oxide materials and, specifically, composite metal oxide ceramic particles in plastic resin matrices.
Various otner devices commonly used in electronic circuits exhibit NLR behavior and have been utilized to provide electrical overstress protection. Typical examples of such devices are semiconductor diodes, transistors, and zener diodes. Specifically, zener diodes have the property of providing nearly infinite resistance until an applied voltage reaches a threshold value and, thereafter, providing rapidly decreasing resistance. Although relatively fast in response time as compared to other overstress protection devices, zener diodes exhibit some capacitance and, thus, provide substantial time delays when encountering EOS transients having rise times measured in nanoseconds or less. Also, practical zener diodes have relatively limited operating regions and lack capacity to handle large amounts of energy.
Zener diodes and other conventionally used EOS protection devices also usually exhibit substantial "overshoot" when encountering rapid transients such as those caused by an EMP. The term overshoot refers to the amount by which transient voltage exceeds the clamping voltage of an overstress protection device prior to the time the device becomes conductive. In diodes for example, overshoot can arise because of inductance in the leads and because of the time required to charge the p-n junction diffusion layers in the diodes. Because circuitry connected to an overstress protection device can be damaged during an overshoot period, overshoot ordinarily should be minimized both in extent and duration.
Spark gap devices also have relatively substantial energy handling capabilities for EOS protection. In operation, spark gaps conduct by forming highly ionized conducting channels with nearly negligible resistance. Because time periods up to several microseconds are required for spark gap devices to absorb enough energy to generate such channels, spark gap devices exhibit substantial overshoot before becoming highly conductive. Also, after a spark gap device becomes conductive at low resistance levels, it may short-out protected circuits.
Thin-film devices for providing EOS protection include various discrete solid-state materials wherein current is conducted in narrow channels. The channels typically are only sub-micron to micron in size and, therefore, can absorb only relatively small amounts of energy before becoming thermally limited. In practice, thin-film devices exhibit substantial overshoot and may lose their recovery properties after reacting to a relatively small number of such transients.
Filters usually comprise combinations of resistors, capacitors, inductors and solid state elements such as diodes, transistors and operational amplifiers. Filters have limited application in protecting against severe EOS transients since, by definition, filters allow certain frequencies to pass while blocking other frequencies. For example, capacitors conduct nigh-frequency signals but block low frequency ones. Because many fast rise time transients contain broad bands of frequencies, including very high or low frequency components, conventional filters provide inadequate EOS protection.
In view of the preceding, it can be appreciated that conventional devices and materials provide inadequate protection when encountering electrical transient disturbances having rise times less than a few nanoseconds and broad frequency spectrums. In addition, individual types of overstress protection devices each have their own shortcomings, particularly failure to recover protective properties after encountering repeated EOS transients having high energies and rapid rise times.